The structural requirements for "essentiality" will be sought through study of the effects of structure of unsaturated and polyunsaturated fatty acids upon relative rates of reactions. The substrates will be the isomers of oleic acid and of linoleic acid. The enzymatic reactions will be chain elongation, chain shortening, desaturation, and incorporation of acyl groups into lipids. Attempts to control metabolic reactions involving unsaturated acids will be made through use of the acetylenic analogs of unsaturated acids, in vivo and in vitro. The nutritive and toxic properties will be tested for a newly discovered group of cyclic products of pyrolysis of fats. The effects of these substances will be tested in whole animals and in vitro. Studies will be made of the involvement of aberrant fatty acid metabolism in the Prader-Willi syndrome of juvenile obesity, Achrodermatitis enteropathica and of EFA deficiency induced by fat-free intravenous feeding.